Overview
This lecture explains ligand-gated ion channels, focusing on how they function as membrane receptors to convert chemical signals into electrical responses in cells.
Types of Membrane Receptors
- Three main membrane receptor categories: ligand-gated ion channels, G protein-coupled receptors, and enzyme-linked receptors.
- This lecture focuses on ligand-gated ion channels (also called ion channel-linked receptors).
Ligand-Gated Ion Channels: Structure and Function
- Ligand-gated ion channels are transmembrane proteins that open or close when a ligand binds.
- Commonly found in excitable cells like neurons because they enable rapid cellular responses to stimuli.
- These channels have a specific site (allosteric site) where the ligand binds, often away from the actual channel pore.
- Binding at the allosteric site causes a conformational change, opening the channel for ions to pass through.
- Ligand binding is specific, following the lock and key or induced fit model.
- Multiple ligand-binding sites can exist on a single channel protein.
Ion Flow and Cellular Response
- When the channel opens, ions (e.g., potassium, sodium, chloride, calcium) rapidly move across the membrane.
- This ion movement causes a change in the electrical properties of the cell, resulting in an intracellular electrical signal.
- The process converts an extracellular chemical signal into an intracellular electrical response, enabling fast cellular actions.
Binding Site Location and Variability
- Most ligand-binding sites are on the extracellular side of the membrane, as membrane receptors primarily detect external signals.
- Rarely, ligand-binding may occur inside the cell.
Distinguishing from Other Ion Channels
- Ligand-gated ion channels are not the same as voltage-gated ion channels, which open in response to membrane potential changes.
- Ligand-gated channels are also different from stretch-activated ion channels, which respond to membrane deformation.
Key Terms & Definitions
- Ligand-gated ion channel — A membrane protein that opens or closes in response to binding a chemical signal (ligand).
- Ligand — A chemical messenger that binds to a receptor or channel.
- Allosteric site — A site on a protein where a molecule binds and induces a functional change at a different site.
- Transmembrane protein — A protein spanning the cell membrane, often acting as a channel or receptor.
- Voltage-gated ion channel — A channel that responds to changes in membrane potential.
- Stretch-activated ion channel — A channel that responds to physical deformation of the cell membrane.
Action Items / Next Steps
- Review the differences between ligand-gated, voltage-gated, and stretch-activated ion channels.
- Understand examples of ions (Na+, K+, Ca2+, Cl-) involved in these channels for next class.